1. Field of the Invention
This invention relates in general to ion sources, and in particular to MALDI mass spectrometry ion sources especially with pulsed dynamic focusing.
2. Background of the Invention
Ionization of chemical species can be accomplished by a variety of methods including matrix-assisted laser desorption/ionization (MALDI), atmospheric pressure (AP)-MALDI, electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI), field ionization, electron ionization, discharge and photoionization. These ionization techniques, when combined with an appropriate mass analyzer or ion mobility spectrometer, yield chemical and structural information about the molecules ionized. One goal of combining an ion source with an instrumental analyzer is to achieve a low limit of detection for a chemical species of interest (i.e., high sensitivity). Another goal is to acquire such information in the fastest time possible (i.e., high throughput).
One combination of ion source and spectrometer is an AP-MALDI mass spectrometry as described by Laiko et al. in Anal. Chem. 2000, 72:652–657; 72:5239–5243; and described in U.S. Pat. No. 5,965,884, the entire contents of which are incorporated herein by reference. As shown in FIG. 1, AP-MALDI system 2 uses a pulsed laser 4 for ionization, at ambient pressures, to create ions for analysis in a mass spectrometer 6. A capillary 8 is used in conventional AP-MALDI MS configurations to transfer ions from the sample target plate 10 (i.e., the ion source) to the mass spectrometer 6.
FIG. 2A is a diagram depicting an enlarged view of an AP-MALDI sampling interface configuration showing a tapered capillary 8, which is itself interfaced to the mass spectrometer 6 by a sampling orifice 9b to an inlet flange 9. The numbers depicted on the figures represent typical values for the dimensions used, and are not intended to specifically restrict the present invention. Capillaries (as shown for example in FIG. 1 and FIG. 2A) can be tapered.
Further, as shown in FIG. 2B, the sampling orifice 9b can utilize sharp tips. However, other sampling inlets 9, as shown in FIGS. 2C and 3, have been used, including arrangements as in FIG. 3 in which a non-parallel sample plate 10 is adjacent to the inlet flange 9. The non-parallel configuration permits laser irradiation to be aligned on-axis with the sampling interfaces, and illustrates one problem overcome by the use of extended capillaries, such as for example capillary 8 shown in FIG. 1, to provide better sample access.
Traditionally, samples were mounted on sample plates 10 and placed close to the inlet flange 9 of the mass spectrometer 6. However, pragmatic considerations such as line-of-sight for laser desorption and imaging drove the development of extended capillary delivery systems such as shown in FIG. 1, in which more space is obtained permitting flexibility in sampling and the sampling from multiple sample plates into one mass spectrometer unit.
To increase ion collection efficiencies in the above shown configurations, electric field extraction techniques were developed. An applied electric field serves to draw ions produced from the sample toward the capillary 8 or the sampling orifice 9b of the mass spectrometer 6. A further enhancement to the electric field extraction techniques has been the application of a pulsed dynamic focusing (PDF) technique which removes the electric field in the sample-to-inlet region, just prior to ions entering the capillary 8 or the sampling orifice 9b. The PDF technique reduces ion losses due to collisions of ions with walls of the capillary 8 or the sampling orifice 9b. This PDF technique as described in U.S. patent application Ser. No. 10/367,917, the entire contents of which are incorporated herein by reference, is often referred to as “timed-extraction” and has also been recently described by Tan et al. in 2004, Anal. Chem., the entire contents of which are incorporated herein by reference.
In brief, the PDF technique permits the use of off-axis ion production techniques from the sampling interface 8, such as for example off-axis laser irradiation, to generate ions from regions not directly in front of the capillary 8 or the inlet flange 9. The PDF technique increases analytical throughput when laser spot sizes are increased. Improvements in throughput with PDF have been demonstrated using AP-MALDI ion trap MS systems with both capillary and conical sampling interfaces. In addition to the higher throughput afforded by the PDF technology, sensitivity was found to be positively correlated with electric field strength.
Ion trajectories and kinetics have been recently modeled for the conventional PDF techniques. Ion simulation typically apply a boundary element method on user-defined geometries, voltage settings and gas flow rates to determine electric field, gas dynamic flow, and ion trajectories. The ion trajectories can be determined based on ion mobility calculations. Such simulations made for example for the configuration shown in FIG. 1 with a tapered extended capillary 8 show that, in a static electric field, ions off-axis from the sampling interface are lost to the walls 12 and tip 14 of the sampling interface (see FIG. 4). Simulations further showed that when PDF was applied to AP-MALDI, off-axis ions are more efficiently collected, since the electric field being terminated before the ions arrive at the walls 12 and tip 14 of the sampling interface does not force the ions onto the walls. Rather, upon termination of the electric field, the ions are entrained in the gas flow entering the mass spectrometer 6.
Further simulations to include ion recombination kinetics to study the relative ion yield associated with different configurations and electric field strengths have determined that the electric field strength directly affects ion signal intensity (see FIG. 5). One possible theory to explain this phenomenon is that positive and negative ions ejected from the sample surface by the laser pulse initially occupy a narrow layer near the target plate. The applied electric field causes these positive and negative ions to move in opposite directions, minimizing ion losses that can result from gas-phase ion recombination and neutralization. From this theory higher electric fields at the site of ionization would improve ionization efficiency and hence sensitivity, as the positive and negative ions are more rapidly separated thus reducing the number of gas-phase ion recombination events.
One potential drawback with the sampling interface designs discussed above is that the electric field may not be optimized at the location of irradiation (i.e. the location of ion generation). Thus, a significant fraction of the ions can recombine or be neutralized. While applying higher voltages to the sample target plate could raise the electric field, arcing and discharge at the higher voltages can limit the upper bound to which the electric field can be adjusted. Furthermore, the electric field in the sampling interface designs may be limited to a range of effectiveness about the sampling interface.